The statement that is true of mean and standard deviation is A. The mean is a parameter, but the standard deviation is an estimator.
How are mean and standard deviation different ?
In a skewed population distribution, the mean is still a parameter, but the standard deviation is an estimator. This is because the mean is a characteristic of the entire population, while the standard deviation is a measure of the variability of a sample.
Since a sample is only a subset of the population, the standard deviation calculated from a sample will only estimate the true standard deviation of the population.